Keep conditional compilation cfg naming consistent with serde crate

The serde crate's build.rs uses names like `core_duration` or
`integer128`, not `has_core_duration` / `has_integer128`.

serde_test/build.rs

@@ -14,7 +14,7 @@ fn main() {
     // #[track_caller] stabilized in Rust 1.46:
     // https://blog.rust-lang.org/2020/08/27/Rust-1.46.0.html#track_caller
     if minor >= 46 {
-        println!("cargo:rustc-cfg=has_track_caller");
+        println!("cargo:rustc-cfg=track_caller");
     }
 }